Project Summary Transcription is the major control point of gene expression and RNA polymerase (RNAP), conserved from bacteria to man, is the central enzyme of transcription. Our long term goal is to understand the mechanism of transcription and its regulation. Determining three-dimensional structures of RNAP and its complexes with DNA, RNA, and regulatory factors, is an essential step. We focus on highly characterized prokaryotic RNAPs. The basic elements of the transcription cycle, initiation, elongation, and termination, were elucidated through study of prokaryotes. A detailed structural and functional understanding of the entire transcription cycle is essential to explain the fundamental control of gene expression and to target RNAP with small-molecule antibiotics. Advances in this understanding are stuck on the difficulty of visualizing transient intermediates that underlie the key transitions between stable states of the transcription cycle, and the difficulty of visualizing complex macromolecular assemblies involved in regulation, structural problems where X-ray crystallography has severe limitations. While the stable RNAP states around the transcription cycle (RNAP catalytic core, RNAP holoenzyme, RNAP holoenzyme open promoter complex, RNAP elongation complex) are relatively well characterized and understood, the transitions between the stable states are poorly understood. Major transitions include: Holoenzyme + promoter DNA ? open promoter complex (initiation) Open promoter complex ? elongation complex (promoter escape, ? dissociation) Elongation complex ? core RNAP + DNA + completed RNA transcript (termination) Each of these transitions are characterized by unstable, transient intermediates that are extremely challenging for structural biology. At every stage of the transcription cycle, RNAP function is modulated by interactions with extrinsic regulatory factors. Assembling and crystallizing transcription complexes containing extrinsic regulators also presents challenges for structural biology. Due to recent advances, cryo-electron microscopy (cryo-EM) now offers a route to structural and mechanistic characterization of these intermediates and large assemblies. We will use cryo-electron microscopy, in combination with X-ray crystallography and other approaches, to exploit this opportunity and provide a complete characterization of the bacterial transcription cycle.